People suffering from visual impairment face many challenges in performing routine daily activities and/or may not be able to fully enjoy the visual aspects of their surroundings. Of particular concern with regard to the current invention are visual impairments caused by damage to the retina, which occur in conditions such as diabetic retinopathy and retinopathy of prematurity.
Diabetic retinopathy is a progressive disease characterized by abnormalities of the blood vessels of the retina caused by diabetes, such as weakening of the blood vessel walls, leakage from the blood vessels, and bleeding and scarring around new vessels. Diabetic retinopathy results in impairment of a person's vision causing severely blurred vision and, potentially, blindness.
Diabetes affects over 16 million Americans. The World Health Organization indicates that diabetes afflicts 120 million people worldwide, and estimates that this number will increase to 300 million by the year 2025. Diabetics are faced with numerous complications including kidney failure, non-traumatic amputations, an increase in the incidence of heart attack or stroke, nerve damage, and loss of vision. Diabetic retinopathy is a form of visual impairment often suffered by diabetics.
Due to significant medical advancements, diabetics are able to live much longer than in the past. However, the longer a person has diabetes the greater the chances of developing diabetic retinopathy. Affecting over 5.3 million Americans, diabetic retinopathy is the leading cause of blindness among adults in the United States. Annually, in the United States, between 12,000 and 24,000 people lose their sight because of diabetes.
While management of diabetic retinopathy has improved, risk of complications, such as loss of visual acuity, loss of night vision and loss of peripheral vision, remains significant and treatment sometimes fails. Currently, laser photocoagulation is the most effective form of therapy for advanced disease. Unfortunately, current treatment options are inadequate and the disease is often progressive even with successful glucose control.
Retinopathy of prematurity (ROP) is a disorder of retinal blood vessel development in the premature infant. Under normal development, blood vessels grow from the back central part of the eye out toward the edges. In premature babies, this process is not complete and the abnormal growth of the vessels proliferate leading to scar tissue development, retinal detachment and possibly complete blindness.
ROP is the major cause of blindness in children under the age of 7. The salient pathological features are neovascularization in the retinal vascular endothelium with edema and breakdown in the blood-retinal barrier (BRB) that leads to hemorrhage, tissue damage and retinal scarring ultimately leads, in the severest cases, to blindness.
Improved care in the neonatal intensive care unit has reduced the incidence of retinopathy of prematurity in moderately premature infants. Ironically, however, increasing rates of survival of very premature infants, who would have had little chance of survival in the past, has increased the occurrence of retinopathy of prematurity. Since these very premature infants are at the highest risk of developing ROP, it is of great concern that the condition may actually be becoming more prevalent again.
For those babies in whom retinopathy progresses, treatment is necessary. Cryotherapy and laser treatment have some effect in advanced stages of the disease, saving a degree of vision in a proportion of the eyes that would otherwise have been blinded, but prevention awaits a better understanding of major causative factors and underlying pathophysiology.
Current research shows promise that the prevention of retinal blood vessel damage, which marks retinopathy, may be achieved by the utilization of certain compounds. It has been demonstrated that, in retinal epithelial cells, glutamine deprivation can lead to upregulation of vascular endothelial growth factor (VEGF) expression (Abcouwer S. et al., “Response of VEGF expression to amino acid deprivation and inducers of endoplasmic reticulum stress,” Invest Ophthalmol Vis Sci, August 2002, pp. 2791–8, Vol. 43, No. 8). Most sick premature infants are deprived of glutamine during the time they receive supplemental oxygen, a known predisposing factor in the development of ROP. The over expression of VEGF during this time period is also thought to be involved in the pathogenesis of ROP. Providing glutamine supplements during this time period could potentially down-regulate VEGF. Arginine is a substrate for the reaction that produces nitric oxide, a very potent vasodilator; vasodilation in retinal blood vessels also prevents neovascularization. Nitric oxide also has numerous other beneficial effects and is now commonly used for treatment of lung disease in critically ill infants.
It is well known that proteins are converted to amino acids in the digestive system and that the resulting amino acids are used by the body for growth and development. Proteins and peptides administered for therapeutic or preventative measures are also well-known. Oligopeptides are better absorbed in the intestines than individual amino acids.
European Patent Application No. 0,182,356 discloses a nutritional composition containing at least one oligopeptide consisting of a dipeptide or a tripeptide wherein the N-terminal amino acid residue is selected from the class consisting of alanine, lysine and arginine.
One group conducting research in this area concluded that glycine is generally superior to other amino acids as the N-terminal amino acid residue in a dipeptide. This superiority was attributed to a greater fraction of such an intravenously administered dipeptide reaching the tissues. S. Adibi et al., Influence of Molecular Structure on Half-life and Hydrolysis of Dipeptides in Plasma: Importance of Glycine as N-Terminal Amino Acid Residue, 35 Metabolism 850, 835 (1986).
Two European patents, 0,087,751 and 0,087,750 disclose water-soluble peptides. The '751 patent discloses a method to parenterally administer low water-soluble amino acids. Two amino acids, tyrosine and cystine, individually have low solubility in water. These amino acids, however, are clinically useful and, therefore, it was desirable to find an effective formulation. The '751 patent describes an infusion method which involves bonding these relatively insoluble amino acids to the amino acid lysine to produce a tripeptide.
The '750 patent discloses the infusion of glutamine as a derivative substituted by α-aminoacyl residues on the α amino group. That is, glutamine is in the “c-terminal” position, in that its alpha amino nitrogen becomes part of the peptide bond with the other amino acid. The preferred dipeptide preparation disclosed in the '750 patent is alanyl-glutamine. The aminoacylation of glutamine is reported to achieve a stabilization of the terminal amide group.
Experiments involving the use of total parenteral nutrition (TPN) containing glycyl-glutamine dipeptides, however, suggest potential adverse effects of the TPN formulation containing glycyl-glutamine (U.S. Pat. No. 5,189,016).
Also, the use of an arginyl-glutamine dipeptide for the prevention of muscle breakdown and microbial infections has recently been described. See, WO 03/017787. These amino acids have also been described in complex compositions (Miyazawa et al. (1976) Journal of Faculty of Fisheries and Animal Husbandry Hiroshima 15(2):161–169; and JP 2119762).
Two commercially available dipeptides of glutamine are DIPEPTIVEN™, which is an alanyl-glutamine (Fresenius Laboratories, Germany) and GLAMIN™(Pharmacia and Upjohn Laboratory, Sweden), which is an amino acid solution containing glycyl-glutamine dipeptide. To this date, there are no published studies of the arginyl-L-glutamine dipeptide.
With the increase of adult onset diabetes, longer life span for diabetics and high rate of survival of very premature infants, many individuals are now at even greater risk for developing retinopathy. Although treatment options, such as laser therapy, exist for both conditions, the results are inadequate and the disease often remains progressive. There remains a great need in the art for compositions which prevent retinal diseases.